Lighting systems based on LEDs are used on an increasing scale.
LEDs have a high efficiency and a long life time. In many lighting systems, LEDs also offer a higher optical efficiency than other light sources. As a consequence LEDs offer an interesting alternative for well-known light sources such as fluorescent lamps, high intensity discharge lamps and incandescent lamps.
The lighting systems based on LEDs often comprise a power supply circuit that supplies power to the LEDs comprised in one or more LED modules that, at least during operation, are electrically connected to output terminals of the power supply circuit. Typically the total current supplied by the power supply circuit depends on the number of LED modules connected to the power supply circuit and more in particular on the desired current that is required by and suitable for each of the LED modules and possibly also on the temperature of the LED modules. Each LED module LM comprised in a LED lighting system called Fortimo manufactured by Philips, which is presently on the market and shown in FIG. 1, comprises a first resistor Rset having a resistance that represents the desired current suitable for the LEDs comprised in the LED module. Each LED module LM also comprises a second resistor NTC with a temperature dependent resistance. When one of these LED modules LM is connected to the power supply circuit PSC, a circuit MC, which is comprised in the power supply circuit PSC, causes a current to flow through the first resistor Rset and another current to flow through the second resistor NTC. The voltages across each of the resistors are measured and the value of the resistance of each of the resistors is determined by the circuit MC from the measured voltage across each of the resistors. From these data, the circuit part MC derives a value for the LED current. A driver circuit DC, which is comprised in the power supply circuit PSC, subsequently adjusts the current supplied to the LED modules to the derived value.
An important disadvantage of this prior art system and method is that three wires are required for connecting the resistors in the LED module with circuitry comprised in the power supply circuit. This makes these existing LED lighting systems rather complex. Furthermore, in case the LED lighting system comprises more than one LED module, this prior art does not allow more than one LED module to be arranged in series or in parallel according to the preference of a user.